Add CSS class names customization.

Hi,

I've made a small change to the code, which allows customization of CSS classes. The idea is to define an optional formCss parameter where class names may be specified by the caller. The patch supports customizing help and error classes.

For example, adding this formCss parameter yo can have help text use class='myHelp' and error text use class='myError' in this form.

```
        var myCss = {
            help: 'myHelp',
            error: 'myError'
        };
        form = AngularForms({ scope: $scope, targetId: 'basicForm', form: SampleForm, formCss: myCss });
        form.inject();
```

Hope you will consider pulling this into the main repository!